Interference resistant local oscillator

ABSTRACT

With some embodiments, a VCO (voltage controlled oscillator) operates at an integer multiple (N) above a desired transmission frequency.

BACKGROUND

High frequency generator circuits such as voltage controlled oscillators(VCOs) are used as local oscillators (LOs) for mixing signals in orderto transmit and/or receive information, e.g., over a wireless or wiredchannel. With higher transmission frequencies (e.g., above 1 GHz), it isdifficult to generate a “clean” high frequency signal. Conventionalmethods of LO generation have large harmonics and spurs that candesensitize the receiver and create spectral compliance challenges forthe transmitter. The noise on the LO can mix with blockers anddesensitize the receiver. It can be even more of a challenge when thevarious transceivers supporting different frequency bands areimplemented in a single chip. For example the unwanted spectral tones ofa local oscillator can interfere with a receiver section of anothertransceiver.

Direct conversion transmitters are desired for low cost, small size andreconfigurable modulation bandwidth. The frequency generator, the VCO,phased locked using a PLL to a crystal oscillator is needed forup-conversion. A VCO running at the transmit frequency, however, cansuffer from frequency pulling by the transmit amplifier. Forhigher-power transmit amplifiers, the high power section may beseparated and shielded (e.g., different chip in a metal-enclosure) fromthe VCO (or VCO could be implemented with a separate shielded chip)circuit to inhibit interference from the high power transmitter back tothe sensitive VCO circuit. Unfortunately, it may be less desirable touse separate chips due to cost reasons, but with present approaches, ifimplemented in a single chip, the VCO may be detrimentally interferedwith by the higher power section. Some transceiver solutions haveinvolved splitting up the conversion into several stages so that thehigher power stage like a transmit power amplifier is at a differentfrequency than the VCO used in the local oscillator. But this requiresmore than one VCO and possibly more than one PLL. The other so calleddirect conversion solutions generate the LO signal indirectly from a VCOthat is offset from the carrier frequency. For example, if a 6 GHztransmission frequency is desired, a VCO may be used to generate a 4 GHzsignal, which can be divided to obtain a 2 GHz component. The 4 GHz and2 GHz signals are then used to attain the desired 6 GHz LO signal.Unfortunately, this requires additional circuitry (mixers, etc.).Because of frequency mixing the LO spectrum may contain unwantedcomponents at 2 GHz, 4 GHz, 8 GHz etc in addition to desired 6 GHzcomponents, requiring in some cases, tuned circuits to reduce theselevels. These circuits as well as the mixer used for LO generation alsoneed large inductors, which consume more power and chip area and makethe solution expensive. Accordingly, new approaches are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings inwhich like reference numerals refer to similar elements.

FIG. 1A is a diagram of a portion of a transmitter with an interferenceresistant local oscillator in accordance with some embodiments.

FIG. 1B is a diagram of a portion of a receiver with an interferenceresistant local oscillator in accordance with some embodiments.

FIG. 2 is a diagram of a voltage controlled oscillator in accordancewith some embodiments.

FIGS. 3A-3D show inductor geometries in accordance with some differentembodiments.

FIG. 4 is a diagram of a transmission system with an interferenceresistant LO in accordance with some embodiments.

FIG. 5 is a diagram of a VCO with a power delivery system in accordancewith some embodiments.

FIG. 6 is a diagram of a computer system with a transceiver having an LOwith interference resistance in accordance with some embodiments.

DETAILED DESCRIPTION

In some embodiments, interference affecting local oscillators can bereduced by using approaches disclosed herein. The approaches may besuitable not only for low power transmitters, but also for medium tohigh power transmitters. With some embodiments, a VCO (voltagecontrolled oscillator) operates at an integer multiple (N) above thedesired transmission frequency. The VCO may be phase locked at theinteger or fractional multiple to a stable reference oscillator (crystaloscillator). The VCO signal may be distributed to multiple radios at thehigher frequency and a divide-by-N frequency divider may be used betweena distribution amplifier and a Tx/Rx so that its mixer(s) operate with alocal oscillator (LO) signal at the target transmission frequency. It iseasier to resist interference in the VCO when it runs at a different(e.g., higher) frequency than that used for transmission in the higherpower section(s). In addition, in some embodiments, techniques such assilicon isolation and the use of noise resistant inductors may alsoand/or alternatively be used in the VCO to make it more resistant tointerference.

FIGS. 1A and 1B show general block portions of a transmitter andreceiver, respectively, with interference resistant local oscillators inaccordance with some embodiments. They are on a chip 100 and may or maynot be part of a common transceiver. They each include a VCO 102,divide-by-N circuit 104, mixer 106, amplifier 108, and antenna 109,coupled as shown. The amplifier 108 corresponds to a higher poweramplifier and/or filter to transmit the signal through the antenna(transmitter 101A case) or to receive it from the antenna 109 (receiver101B case). In the case of the transmitter, a baseband (BB) signal ismixed with the local oscillator signal, LO (Fo), generating a signal(Tout(Fo)) at the output of the amplifier 108. In the case of thereceiver 101B, the received transmission signal (Tin(Fo)) is mixed withthe local oscillator signal (LO(Fo)), generating the baseband signal(BB).

The VCO is made to generate a signal at a frequency that is N timeslarger than that desired for transmission. (It should be noted that theterm transmission is intended to refer to both transmitting andreceiving. That is, a signal at a transmission frequency may be receivedor transmitted. In addition, along these lines, the transmissionfrequency refers to the frequency used at a mixer or modulator for ahigher power stage. It encompasses schemes whereby the transmittedsignal may be spread or modulated, albeit possibly slightly in somecases, depending upon the utilized transmission scheme, e.g., WiMax,WiFi, GPS, etc.) A divide-by-P circuit may be interposed between the VCOand a PLL (phase locked loop), referenced at Fc (e.g., from a crystalclock generator), to control the VCO to run at PFc (PFc=NFo). Thedivide-by-N block 104 divides the VCO generated signal (which is at NFo)to be at the desired transmission frequency (Fo).

In some embodiments, as discussed herein, the VCO may be made to be moreresistant to interference from the transmitted signal. For example,silicon isolation methods like multiple guard rings interleaved withhigh-resistance channels may be employed, numerous different down-bondsmay be used for these guard rings and the power supplied to the VCO maybe provided in a manner so as to enhance VCO stability. In addition,since the VCO is to operate at a relatively high frequency, whenemploying a tank circuit configuration, the utilized inductor can bemade smaller and as taught herein, can be made to be more resistant toelectro-magnetic interference.

FIG. 2 generally shows an exemplary tank-based VCO circuit for use withthe circuits of FIG. 1. It comprises cross-coupled transistors N1 andN2, variable capacitor C, and inductor L, coupled as shown. Supply power(VDD) for the VCO may be provided through a center tap of the inductor.While any suitable way may be used to control the generated frequencybased on a control signal (voltage or current, even though the circuitis referred to as a voltage controlled oscillator), a varactor (variablecapacitor, typically voltage controlled) is commonly used forcontrolling the capacitance in the tank and thus the generatedfrequency. (The generated frequency in a tank circuit is normally1/[2π√(LC)].) Thus, by operating the VCO at a higher frequency, theinductor and/or capacitor blocks can be made smaller. In the case of theinductor, this can enable more flexible inductor designs for resistinginterference.

FIGS. 3A to 3D show different embodiments for implementing planar,interference resistant inductors (integrated on a chip) that may beused. The inductors in FIGS. 3A-3C have terminals, T1 and T2, and aregenerally designed to have somewhat symmetrical loops (Loop 1 and Loop2). They may also have a center tap located between the terminals oneither loop. The inductor of FIG. 3D is a coupled inductor with primaryand secondary windings 336 and 338. (Also shown in this figure are blocksections, 332 and 334, where capacitor banks and active circuitry forthe VCO may be located relative to each other in a chip.) The inductorsmay be disposed (or oriented) so that interference (that would otherwisebe picked up and incorporated into the electrical signal in theinductor) is electrically cancelled in the loops, akin to interferenceencountering twisted differential pair lines. That is, the radiationreaching the inductor couples to two (or more) symmetrical structuresand cancel out, at least to a given extent, depending on the circuitlayout

By optimizing the relative orientation of the inductor to the directionof magnetic interference, maximum cancellation of interference can beachieved. For example, with the inductor of FIG. 3A, interferencehitting the inductor between the loops, e.g., along an axis dividing theloops, will be cancelled the most since electrical signals from theinterference are generated in opposite polarities in the loops andcancel each other to the extent that the magnetic field is identical inthe two loops. It was contemplated that using multiple loops, e.g., fouror more, akin to the inductor of FIG. 3C, would yield betterinterference resistance since the quality of cancelling would be lessdependent on the direction of the oncoming interference. However, it wasexperimentally determined that disadvantages resulting from additiveinterference outweighed the benefits of reducing orientationsensitivity. Regardless, there are many different geometries that may beemployed, providing improved interference resistance for a VCO and arecontemplated within the scope of the invention. Also, multiple suchinductors may be used if necessary in a given VCO circuit. Use of thesestructures in appropriate orientation may allow the blocks to be closelyspaced to minimize area without compromising performance.

The inductor(s) can be further protected from magnetic interferenceusing shielding rings around them that allow current flow in a circularloop. For example a low impedance circular loop may be used to enclosethe inductor and will minimize the radiation pickup. However, it shouldbe remembered that the use of such a closed conductive path outside theinductor may degrade the Q of VCO tank.

A transformer (or coupled inductor) such as that shown in FIG. 3D canalso be used to reduce coupling while assisting in frequency tuning. TheVCO may comprise an inductor, mostly constituted from the primarywinding 336, and cap bank and active devices 334. The secondary winding338 may be connected to another switched cap bank 332, or a simpleswitch, to provide a closed path, or a switched inductance, to increasethe tuning range of the main tank circuit while enhancing immunity toradiation pickup

FIG. 4 shows a diagram of a transceiver (transmitter and/or receiver)system with an interference resistant local oscillator including aninterference resistant VCO 102. The VCO 102, along with phase frequencydetector (PFD) 404, charge pump 406, buffer (amplifier) 410, anddivide-by-P block 412 make up a phase locked loop referenced off of acrystal clock 402. (For simplicity, other circuit elements have beenomitted. for example, components could be included for adjusting thefrequency (Fc), e.g., for carrier recovery or aligning the crystalreference frequency to base-station frequency. Moreover, any suitablePLL type, e.g., integer or fractional type, may be employed. Sigma-deltamodulation may be used inside the loop to reduce the noise. In addition,a digital or analog loop filter may be used in the PLL, or an alldigital PLL could be employed. The buffer 410 provides the signalgenerated from the VCO (with frequency PFc) to a high frequencyTransmission line (T line) and distribution amplifier 414, which feedsthe higher frequency signal to one or more transceivers, Transceiver 1through Transceiver M. The transceivers each include a divide-by-N block416, mixer 418, and higher power amplifier 420 to mix a divided downsignal (at frequency Fo) with a baseband (BB) signal. The distancestraversed by signals between the VCO 102 and mixer 418 may be relativelylong and thus it is advantageous to route the signal(s) from the VCO athigher frequencies and then divide them down to the desired transmitfrequency once at the transceiver. The distance between the divide-by-Nblock 416 and mixer 418 should also be minimized.

With additional reference back to FIGS. 1A and 1B, Placing thedivide-by-N block 416 close to the up/down conversion mixer, 418,reduces the on-frequency coupling to the substrate from the divider 416output to a low noise amplifier, e.g., LNA 118 from FIG. 1B, which couldinterfere with Rx operation. The higher power transmissions willgenerate undesirable noise that leaks back to the VCO. This includes thenoise, e.g., harmonics, generated in the high power section 108. Strongsecond harmonics from the supply node of high power amplifier 108 canalso couple to the VCO. The sensitivity of the VCO to the noise frompower amp. 108 may be reduced by reducing the magnetic flux at the poweramplifier, as well as by designing the VCO to be resistant to radiationpickup at that frequency. In addition, the high frequency noise atseveral points in the circuit may be suppressed by using reactiveelements like capacitors or LC series resonant filters. These reactiveelements and filters may be designed using lumped elements ordistributed transmission lines. Fortunately, it is easier to filter outthe lower frequency noise which is generated due to the modulationenvelope of the PA.

In addition, the VCO 102 may include one or more features to make itmore resistant to interference produced at the higher powertransceivers. For example, filtering at various, more sensitive nodes inthe VCO may be employed. Also, silicon isolation may be used. This mayinvolve using more than one down-bond for the VCO and multiple guardrings can be used to minimize the substrate pickup. Guard rings shouldbe tied to a clean ground using multiple down-bonds. When multiple guardrings are used, they should terminate at different ground and supplyconnections in order to avoid corrupting the quieter ring from gettingcorrupted by the noisier ground.

Interference resistant inductors, as previously discussed, can also beused to reduce magnetic coupling. Furthermore, as discussed below withreference to FIG. 5, a power supply with a high Power Supply RejectionRatio (PSRR) for the VCO may be used to enhance VCO frequency and phasestability’ (phase noise performance) and operation.

The value of frequency division ratio N may be any suitable number,e.g., 2, 3 or some other number. For example, in an embodiment of a WiFiradio, with a 5.5 GHz transmission frequency (Fo), N could be 2 and thusthe VCO could be operated at 11 GHz. An appropriate value for N may beselected based on given design requirements. The value should be chosenthat makes a good compromise between operating performance parametersand the size of inductor. A small value of N implies an easier design,but it generally requires a larger chip area due to a larger size neededfor the inductor. A larger inductor size also implies that more lines offlux can be intercepted and hence result in a stronger pickup forinjection pulling in the VCO. In some embodiments, a large N may improvethe VCO phase noise by 6 dB for every divide-by-2 operation. Along withVCO phase noise reduction, noise picked up by the VCO, which may appearas FM sidebands, also may be reduced by the same factor. However, toolarge a value of N results in a very high VCO frequency operation, whichmay reduce the tuning range, although the tuning range may becompensated by using multiple VCOs. Since the inductor size is verysmall at high frequencies, multiple VCOs do not necessarily have toconsume too much chip area and the method can provide a clean localoscillator. In addition, a smaller N value gives lower improvement ininterference resistance from the power section. With that being said, insome embodiments, a value of 2 may be perfectly suitable.

FIG. 5 shows a VCO 502 that may be used as a VCO 102 in FIGS. 1 or 4.The VCO comprises transistors N1, N2, inductor L, capacitor bank 503,voltage controlled varactor 505, and LC tuned filter 507, all coupledtogether as shown. The VCO is powered using a combination of regulatorsto provide a suitable VDD for enhancing the stability and noiseresistance of the VCO. The supply comprises a pre regulator 522, whichsupplies a regulated voltage supply to a bandgap reference (VR) 524 (theblock 524 also contains a PTAT current source used to generate areference voltage by supplying a regulated current to a replicatransistor N3) and to a high bandwidth regulator 526. The bandgapreference VR 524 is made to provide a highly accurate reference voltagethat is reasonably insensitive to environmental changes such as changesin temperature. For example, it could include biasing circuits likeregulators. A dedicated Band-gap reference circuit can be used to ensurenoiseless operation. A Proportional To Absolute Temperature (PTAT)circuit may be used to cancel the changes in the circuit behavior overtemperature changes, thus stabilizing the operation and ensuring startupover temperature variations. The reference regulator is used to not onlyprovide a precision regulated reference voltage to the high bandwidthregulator 526, but also, it provides the various bias control signals tothe VCO. For example, the control logic of the capacitor bank 503 may becoupled to a bias signal for controlling its capacitance. The referencevoltage provided to the high bandwidth regulator 526 is generatedthrough a diode-connected replica transistor N3, which is made toreasonably match and be proximal to the transistors N1, N2 used in theVCO. This makes the VCO functionality less sensitive to processdeviations and the like.

The higher bandwidth regulator is made to have a sufficiently highbandwidth substantially more than the modulation bandwidth of thetransmitter to have sufficient responsiveness for supplying the VCOrelative to the frequency that it is generating. The regulator should beinternally compensated in order to avoid high frequency pickups such asfrom bond-wire coupling, and it should be made low noise to keep thephase noise low. With the use of separate regulators, overall powersupply noise rejection can be achieved, e.g., via the referenceregulator and pre regulator 522, without having to suffer limitedresponsiveness since it is attained through the separate higherbandwidth regulator 526. The entire bias circuit is dedicated to the VCOcircuit and is enclosed along with the VCO within the previouslymentioned guard rings to avoid picking up the noise through thesubstrate.

The varactor for the VCO should be designed to be small enough to have areduced VCO tuning sensitivity (also called KVCO) such that any noisecoupling to a tuning line has a minimal impact. A small varactor sizewill provide a small, non-linear capacitor and hence lower phase noise.However, too small a varactor can result in two issues. First, anythermal gradients during a Tx slot may change the operating frequency,which is corrected by the PLL by changing the tuning voltage, a smallvaractor can have a limited frequency correction and may unlock the PLLunless a new capacitor value is switched in the capacitor bank. Second,a very small varactor may not have enough tuning range to providesufficient overlap in the fixed (e.g., switchable) capacitor(s) of thetank. Further, the control logic to the VCO block is passed throughlogic buffers which are located within the guard rings along with theVCO, and these logic buffers are powered by the clean power supplygenerated by the dedicated biasing circuits within the guard ring.

With reference to FIG. 6, one example of a portion of a computingplatform (e.g., computing system such as a mobile personal computer,PDA, cell phone, or the like) is shown. The represented portioncomprises one or more processors 602, hub functionality circuit 604,memory 606, wireless network interface 608, and an antenna 609. Theprocessor 602 is coupled to the memory 606 and wireless networkinterface 608 through the hub functionality block 604. The hubfunctionality may comprise one or more circuit blocks to perform variousinterface control functions (e.g., memory control, graphics control, I/Ointerface control, and the like. These circuits may be implemented onone or more separate chips and/or may be partially or wholly implementedwithin the processor 602.

The memory 606 comprises one or more memory blocks to provide additionalrandom access memory to the processor 602. It may be implemented withany suitable memory including but not limited to dynamic random accessmemory, static random access memory, flash memory, or the like. Thewireless network interface 608 is coupled to the antenna 609 towirelessly couple the processor 602 to a wireless network (not shown)such as a wireless local area network or a cellular network. it includesone or more transceivers 611 with interference resistant VCOs asdiscussed herein.

The computer platform may implement a variety of different computingdevices or other appliances with computing capability. Such devicesinclude but are not limited to laptop computers, notebook computers,personal digital assistant devices (PDAs), cellular phones, audio and/oror video media players, and the like. It could constitute one or morecomplete computing systems or alternatively, it could constitute one ormore components useful within a computing system. In addition, the blockdiagram could correspond to a system on chip (SOC) platform implementedon a single chip or in a single package.

In the preceding description, numerous specific details have been setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known circuits, structures and techniques may have not been shownin detail in order not to obscure an understanding of the description.With this in mind, references to “one embodiment”, “an embodiment”,“example embodiment”, “various embodiments”, etc., indicate that theembodiment(s) of the invention so described may include particularfeatures, structures, or characteristics, but not every embodimentnecessarily includes the particular features, structures, orcharacteristics. Further, some embodiments may have some, all, or noneof the features described for other embodiments.

In the preceding description and following claims, the following termsshould be construed as follows: The terms “coupled” and “connected,”along with their derivatives, may be used. It should be understood thatthese terms are not intended as synonyms for each other. Rather, inparticular embodiments, “connected” is used to indicate that two or moreelements are in direct physical or electrical contact with each other.“Coupled” is used to indicate that two or more elements co-operate orinteract with each other, but they may or may not be in direct physicalor electrical contact.

The term “PMOS transistor” refers to a P-type metal oxide semiconductorfield effect transistor. Likewise, “NMOS transistor” refers to an N-typemetal oxide semiconductor field effect transistor. It should beappreciated that whenever the terms: “MOS transistor”, “NMOStransistor”, or “PMOS transistor” are used, unless otherwise expresslyindicated or dictated by the nature of their use, they are being used inan exemplary manner. They encompass the different varieties of MOSdevices including devices with different VTs, material types, insulatorthicknesses, gate(s) configurations, to mention just a few. Moreover,unless specifically referred to as MOS or the like, the term transistorcan include other suitable transistor types, e.g., junction-field-effecttransistors, bipolar-junction transistors, metal semiconductor FETs, andvarious types of three dimensional transistors, MOS or otherwise, knowntoday or not yet developed.

The invention is not limited to the embodiments described, but can bepracticed with modification and alteration within the spirit and scopeof the appended claims. For example, it should be appreciated that thepresent invention is applicable for use with all types of semiconductorintegrated circuit (“IC”) chips. Examples of these IC chips include butare not limited to processors, controllers, chip set components,programmable logic arrays (PLA), memory chips, network-chips, and thelike.

It should also be appreciated that in some of the drawings, signalconductor lines are represented with lines. Some may be thicker, toindicate more constituent signal paths, have a number label, to indicatea number of constituent signal paths, and/or have arrows at one or moreends, to indicate primary information flow direction. This, however,should not be construed in a limiting manner. Rather, such added detailmay be used in connection with one or more exemplary embodiments tofacilitate easier understanding of a circuit. Any represented signallines, whether or not having additional information, may actuallycomprise one or more signals that may travel in multiple directions andmay be implemented with any suitable type of signal scheme, e.g.,digital or analog lines implemented with differential pairs, opticalfiber lines, and/or single-ended lines.

It should be appreciated that example sizes/models/values/ranges mayhave been given, although the present invention is not limited to thesame. As manufacturing techniques (e.g., photolithography) mature overtime, it is expected that devices of smaller size could be manufactured.In addition, well known power/ground connections to IC chips and othercomponents may or may not be shown within the FIGS, for simplicity ofillustration and discussion, and so as not to obscure the invention.Further, arrangements may be shown in block diagram form in order toavoid obscuring the invention, and also in view of the fact thatspecifics with respect to implementation of such block diagramarrangements are highly dependent upon the platform within which thepresent invention is to be implemented, i.e., such specifics should bewell within purview of one skilled in the art. Where specific details(e.g., circuits) are set forth in order to describe example embodimentsof the invention, it should be apparent to one skilled in the art thatthe invention can be practiced without, or with variation of, thesespecific details. The description is thus to be regarded as illustrativeinstead of limiting.

1. A chip, comprising: a VCO to generate a signal; and a frequencydividing circuit to provide a reduced frequency version of the signal toa transmit mixer followed by a power amplifier that is on the same dieas the VCO.
 2. The chip of claim 1, in which the frequency dividingcircuit is a divide-by-N circuit.
 3. The chip of claim 1, in which thepower amplifier is to generate an OFDM output transmission.
 4. The chipof claim 1, in which the VCO comprises a tank with a varactor to controlthe frequency of the VCO.
 5. The chip of claim 1, further comprising apower regulator comprising a bandgap reference regulator and a higherbandwidth regulator to provide power to the VCO.
 6. The chip of claim 5,in which the reference regulator generates a reference voltage off of atransistor matched with and proximal to one or more transistors in theVCO.
 7. The chip of claim 1, in which the value of frequency division istwo.
 8. The chip of claim 1, in which the VCO is to operate at afrequency in excess of 10 GHz.
 9. The chip of claim 1, in which the VCOcomprises a noise resistant inductor.
 10. The chip of claim 9, in whichthe noise resistant inductor comprises one or more pairs ofsubstantially symmetrical loops.
 11. The chip of claim 1, in which theVCO provides its generated higher frequency signal over a distributionamplifier to multiple mixers at remote distances from the VCO on thedie.
 12. A method comprising: generating a signal, using a VCO, at afrequency N times larger than a desired transmission frequency; anddividing the signal by N remotely from the VCO on a common chip andproviding it to a mixer for an output wireless transmission stageamplifier that is on the same chip as the VCO.
 13. The method of claim12, in which N is two.
 14. The method of claim 12, in which the signalis generated using a VCO with a noise resistant inductor.
 15. The methodof claim 14, comprising providing silicon isolation at the VCO.
 16. Themethod of claim 12, in which the VCO is operated in excess of 10 GHz.17. A computer system, comprising: a processor; and a wireless interfacecoupled to the processor to communicatively link the processor to awireless network, the wireless interface comprising at least one localoscillator on a chip, the local oscillator including a VCO to provide asignal at N times a transmission frequency, and a divide-by-N circuit todivide the signal from the VCO by N to be provided at the transmissionfrequency to a mixer for a higher power stage that is on the same die asthe VCO.
 18. The system of claim 17, in which the VCO comprises a tankwith a varactor to control the frequency of the VCO.
 19. The system ofclaim 17, further comprising a power regulator comprising a referenceregulator and a higher bandwidth regulator to provide power to the VCO.20. The system of claim 19, in which the reference regulator generates areference voltage off of a transistor matched with and proximal to oneor more transistors in the VCO.
 21. The system of claim 17, in which thevalue of N is two.
 22. The system of claim 17, in which the VCO is tooperate at a frequency in excess of 10 GHz.